Reactive orthotropic lattice diffuser for noise reduction

ABSTRACT

An orthotropic lattice structure interconnects porous surfaces of the flap with internal lattice-structured perforations to equalize the steady pressure field on the flap surfaces adjacent to the end and to reduce the amplitude of the fluctuations in the flow field near the flap end. The global communication that exists within all of the perforations provides the mechanism to lessen the pressure gradients experienced by the end portion of the flap. In addition to having diffusive effects (diffusing the incoming flow), the three-dimensional orthogonal lattice structure is also reactive (acoustic wave phase distortion) due to the interconnection of the perforations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.13/417,347 titled “Flap Side Edge Liners for Airframe Noise Reduction,”filed on Mar. 12, 2012. Further, this application is related toco-pending U.S. patent application Ser. No. 13/417,349, filed Mar. 12,2012, titled “Landing Gear Door Liners for Airframe Noise Reduction,”and U.S. patent application Ser. No. 13/417,351, filed Mar. 12, 2012,titled “External Acoustic Liners for Multi-Functional Aircraft NoiseReduction.” This application further claims the benefits of U.S.Provisional Application Nos. 61/451,727, filed on Mar. 11, 2011;61/451,730 filed on Mar. 11, 2011; 61/451,735 filed on Mar. 11, 2011;and 61/597,282, filed on Feb. 10, 2012, the entire contents of allincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefore.

FIELD OF THE INVENTION

The present invention relates to reduction in aircraft noise, and inparticular to aircraft noise generated by the airframe of the aircraftduring operation.

BACKGROUND OF THE INVENTION

One of the more important constraints to the continued growth of airtraffic is the related concern regarding aircraft noise. This concernhas resulted in increasingly stringent noise restrictions for airports.During aircraft take-off, the dominant aircraft noise source isgenerally the propulsion noise from the engines of the aircraft. Duringaircraft approach and landing, airframe noise becomes a prominentcomponent on par with the engine noise. This airframe noise is caused bythe interaction of the unsteady and typically turbulent airflow with theaircraft structures. The sound radiated from the side edge of apartial-span flap is one of the major contributors to airframe noiseduring aircraft approach and landing.

Acoustic liners have been used to absorb propulsion noise withinaircraft engine nacelles. Acoustic liners with variable-depth chambersto achieve broadband noise attenuation have been known for some time.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a flap of the type that ismovably connected to an aircraft wing to provide control of an aircraftin flight. The flap includes a flap structure defining inboard andoutboard ends, and forward and rearward portions in the form of leadingand trailing edges extending between the inboard and outboard ends. Theflap structure may be elongated or it may comprise a relatively shortstructure such as a flaperon. The elongated flap structure defines anupper side extending between the inboard and outboard ends and betweenthe forward and rearward portions of the elongated flap structure. Theupper side of the elongated flap structure generally faces upwardly inuse, and the lower side generally faces downwardly in use. The inboardend defines an inwardly-facing inboard end surface, and the outboard enddefines an outwardly-facing outboard end surface. The upper and lowersides define inboard and outboard end portions adjacent the inboard andoutboard ends, respectively, that tend to experience significant localhydrodynamic fluctuations associated with the scrubbing of unsteady flowover the inboard and outboard end surfaces. These fluctuations tend togenerate noise to form a noise source.

At least a portion of the inboard and/or outboard end surfaces, and/orthe inboard and outboard end portions of the upper or lower side of theflap include a substantially porous surface that is acousticallyconnected to chambers (or passageways) that are disposed within theelongated flap structure. Noise generated by the noise source enters theinternal chambers through the openings, and a portion of that noise isabsorbed to thereby reduce the amount of noise that would otherwise betransmitted to the surrounding environment. If the chambers aresufficiently small in diameter, are terminated within the flapstructure, and are not allowed to communicate with other internalchambers, the structure forms a local-reacting liner. If the chambersare large in diameter, or are interconnected, the structure forms anextended-reacting liner. Thus, noise absorption is achieved via all ofthe conventional acoustic liner mechanisms (e.g. viscothermal losseswithin the chamber, vortex shedding). The impedance boundary conditionpresent at the surface of the porous surface also provides apressure-release condition that may reduce the local hydrodynamicfluctuations, thereby inhibiting the noise generation process. In afurther embodiment, an orthotropic lattice structure interconnectsporous surfaces of the flap with internal lattice-structuredperforations to equalize the steady pressure field on the flap surfacesadjacent to the edge and to reduce the amplitude of the fluctuations inthe flow field near the flap edge.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially fragmentary isometric view of an aircraft havingwings and wing flaps;

FIG. 2 is a partially fragmentary enlarged isometric view of a portionof the wing flap of the aircraft of FIG. 1;

FIG. 3 is a cross-sectional view taken along the line III-III; FIG. 1,showing one possible configuration of the acoustic chambers;

FIG. 4 is a cross-sectional view of a flap showing another possibleconfiguration of the internal acoustic chambers;

FIG. 5 is a cross-sectional view of a wing flap according to anotheraspect of the present invention;

FIG. 6 is an isometric view of a wing flap for the aircraft of FIG. 1.

FIG. 7 is an outline of a flap end illustrating the dimension t_(max).

FIG. 8 is a cross-sectional view of a flap showing the internalorthogonal lattice-structure perforations;

FIG. 9 is a partial enlarged view of the internal orthogonallattice-structured perforations;

FIG. 10 is an illustration of circular perforation diameter and spacing;and

FIG. 11 is an illustration of a perforation intersection; and

FIG. 12 is a graph illustrating computational results.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of description herein, the terms “upper,” “lower,” “right,”“left,” “rear,” “front,” “vertical,” “horizontal,” and derivativesthereof shall be related to the invention as oriented in FIG. 1.However, it is to be understood that the invention may assume variousalternative orientations, except where expressly specified to thecontrary. It is also to be understood that the specific devices andprocesses illustrated in the attached drawing, and described in thefollowing specification are simply exemplary embodiments of theinventive concepts defined in the appended claims. Furthermore,references to specific items or features (e.g. a wing structure, leadingedge slat, and slat cove filler) shall not be construed as limiting theitem or feature to one-piece or multi-piece items or features. Hence,specific dimensions and other physical characteristics relating to theembodiments disclosed herein are not to be considered as limiting,unless the claims expressly state otherwise.

An aircraft 1 (FIG. 1) includes a fuselage 2, wings 3, and horizontaland vertical rear stabilizers 4 and 5, respectively. The aircraft 1 alsoincludes turbofan engines 6 or other propulsion system such as openrotor engines. The fuselage 2, wings 3, stabilizers 4 and 5, and engines6 may be of a known type, and these components of aircraft 1 willtherefore not be described in more detail herein. The left half ofaircraft 1 is shown in dashed lines in FIG. 1, but it will be understoodthat the left and right halves of aircraft 1 are mirror images of oneanother such that only the right half is described in detail herein.

Wing 3 may include upper and lower surfaces 11 and 12 extending betweena leading edge 13 and a trailing edge 14. The aircraft may include anelongated flap structure 10 that is movably connected to the wing 3adjacent trailing edge 14. The flap 10 is movably interconnected withthe wing 3 by connectors 15 to provide control of aircraft 1. Ingeneral, powered actuators (not shown) or the like may be utilized toprovide movement of flap structure 10 relative to the wing 3. Theconnecting structures 15 and powered actuators may also be of aconventional type, and are not therefore described in detail herein.

The flap structure 10 includes inboard and outboard ends 20 and 22,respectively, and forward and rearward portions 24 and 26, respectively.The flap structure 10 further includes an upper side 28 having an uppersurface 29 and a lower side 30 having a lower surface 31 (see also FIG.3).

Wing design for conventional transport aircraft is driven largely bycruise efficiency, i.e., the need to generate adequate lift with minimaldrag for level flight at high speeds. Conventional high-lift systems(leading-edge slats and trailing-edge flaps) are designed to augmentlift and improve stall characteristics at the low landing speedsrequired under many circumstances. These multi-element airfoil systemsincrease the effective chord (streamwise dimension) of the wing and thusits effective area. The major effect of the multi-element airfoilarrangement is to generate a much larger pressure difference (lift)between the upper (suction) and lower (pressure) surfaces than would bepossible utilizing a single airfoil element. However, the multi-elementimplementation of the high-lift system presents many discontinuities andother unfavorable geometric characteristics to the flow. These geometricfeatures cause considerable unsteadiness in the flow, which is theprimary source of aeroacoustic noise.

The principal geometric features responsible for producing flowunsteadiness around flap 10, and thus noise, are the inboard andoutboard edges 21 and 23, respectively, at the inboard and outboard ends20 and 22, respectively. Computational results display strong suctionpeaks at the inboard and outboard edges 21 and 23, respectively of flap10. The suction peaks are attributed to the presence of strongstream-wide vortices. Existence of a strong pressure differentialbetween the bottom and top surfaces of the flap results in the formationof a complex dual-vortex system. Specifically, near the flap leadingedge, the boundary layer on the bottom surface separates at the sharpcorner and rolls up to form a stronger of the two vortices. Similarly,the thin boundary layer on the side edge separates at the sharp topcorner and forms what is initially the weaker of the two vortices. Bothvortices gain strength and size along the flap chord because of thesustained generation of vorticity. Downstream of the flap mid-chord, theside vortex begins to interact and merge with the vortex on the topsurface. Eventually, a single dominant stream-wise vortex is formed.

Considerable flow unsteadiness (noise sources) is produced during theshear layer roll up, vortex formation and vortex merging process as wellas by the interaction of the vortices with the sharp corners at the flapedge. The multi-element airfoil reverts to a smooth single-elementprofile during the cruise phase of flight to reduce wing drag. Incurrent practice, the multiple airfoil elements are nested together in aretracted position.

The present invention includes a method and structure that reduces thissource of airframe noise without compromising cruise efficiency, lift,and stall characteristics at landing. As discussed in more detail below,one aspect of the present invention is the use of acoustic liners thatare imbedded within the flap structure 10 to target the propagationphase of acoustic disturbances generated elsewhere. It also targets thevery process of noise generation via interaction of the unsteady flowwith the flap side edges 21 and 23. By limiting the control action tofluctuations within the flow, the gross aerodynamic characteristics areleft unaltered and, hence, the expected aerodynamic penalty is small ornone at all. However, the limited volume within the flap edge creates asignificant packaging challenge. In addition, the broad frequency range(potentially greater than 3.5 octaves) to be attenuated createsadditional challenges.

Known technology used in current commercial aircraft to reduce noisegenerated at the flap side edges involves implementation of “clean”configurations. Prior concepts include the application of fences,continuous moldline link, porous treatment, and brushes at the flap sideedges. Given the critical functionality of aircraft flaps in the controlof the aircraft, reconfiguration of this system may be problematic. Theknown concepts offer a range of noise reduction potentials, but theyalso have disadvantages ranging from weight penalties, parasitic dragpenalties during cruise, and/or a loss of aerodynamic efficiency.

With further reference to FIG. 2, a flap structure according to oneaspect of the present invention includes an acoustic liner 34 on upperside 28 of flap 10 and/or an acoustic liner 36 on end 20 (or 22), and/oran acoustic liner 38 on a lower side 30 of flap 10 (see also FIG. 4).The end portion 40 of the flap 10 comprises the region of the flap atwhich unsteady flow generates noise. In FIG. 2, the end portion 40 isseparated from a central portion 44 of flap 10 by a line designated 42.It will be understood, however, that the end portion 40 forming a sourceof noise does not necessarily have a clear boundary line 42, and the endportion 40 may have different sizes and shapes, with no distinctboundary relative to central portion 44 of flap structure 10. Also, theend portion 40 at which unsteady flow occurs may also change in widthfrom the leading edge 13 to the trailing edge 14. Still further, theareas at the ends of flap 10 that experience unsteady flow may change asaircraft velocity, altitude, and other such operating parameters vary.In a typical commercial aircraft, the end portion 40 is approximatelytwo to ten inches wide (measured from the end of flap 10).

The acoustic liner 34 comprises a thin sheet of material 46 having aplurality of perforations 47 arranged in a substantially uniformrepeating pattern to thereby define a porous upper surface portion 48.The perforations 47 may also be arranged in a non-uniform pattern thatmay or may not be repeating. Porous upper surface 48 may be formed by athin sheet of material having a plurality of perforations 51, or it maycomprise a mesh, or it may be defined by the upper most surfaces of wing3 adjacent to the openings of the internal chambers (or passageways) 60(FIGS. 3 and 4) described in more detail below.

Similarly, acoustic liner 36 at end 22 (or end 20) may comprise a sheetof material 50 having a plurality of perforations 51 defining a porousouter surface 58. Surface 58 may be substantially planar, or it may becurved. In general, surface 58 may have a shape/contour that issubstantially identical to the shape/contour of the original end surfaceof flap 10 for a particular aircraft as originally designed by anaircraft manufacturer. Alternately, surface 58 may be specificallycontoured to optimize aerodynamics and/or noise reduction taking intoaccount changes to airflow resulting from the presence of one or moreacoustic liners according to the present invention. Similarly, acousticliner 38 on the bottom 30 of flap 10 may comprise a thin sheet ofmaterial 54 having a plurality of perforations 55 that permit sound toenter acoustic internal chambers 60 formed in body 18 of flap structure10. The flap 10 may include only a top acoustic liner 34, or it mayinclude only an end acoustic liner 36, or only a bottom acoustic liner38, or it may include any combination of the liners 34, 36, and 38,depending upon the requirements of a particular application. Also, theacoustic liners 34, 36, and 38 may cover substantially all of the upperand lower surfaces at end portion 40, or they may extend over only asegment of end portion 40 of flap structure 10. Similarly, the endacoustic liner 36 may cover substantially all of the end 20 (or 21), orit may cover only a portion thereof. The acoustic liners 34 and/or 36and/or 38 are designed to absorb sound having a specific acousticfrequency profile, and to provide optimum aerodynamic characteristics.

With further reference to FIG. 3, acoustic liner 34 on upper side 28 offlap 10 may comprise a plurality of internal acoustic chambers 60A-60Ehaving openings 62A-62E at a porous upper surface 48 that form outerends of chambers 60A-60E. In the liner 34 shown in FIG. 3, the acousticchambers 60A-60E each include an outer portion 64A-64E, respectively,that extends inwardly in a direction that is generally transverse orperpendicular to porous upper surface 48. Also, chambers 60A and 60Bhave inner end portions 66A and 66B, respectively, that extendtransverse relative to the outer portions 64A and 64B. A center chamber60C, however, only includes a straight portion 64C. Acoustic chambers60D and 60E also include inner, transversely-extending inner endportions 66D and 66E. The inner end portions generally extend at anangle in the range of about 30°-90° relative to outer portion 64A-64E.In general, the key constraint for the orientation of the internalchambers is the requirement that all of the chambers must fit within thevolume 10 of the flap. The required chamber length can be realizedutilizing straight, bent, L-shaped, or U-shaped configurations. Theshape or combination of shapes chosen is dependent upon the lengthsrequired to achieve optimal sound absorption (or pressure release) aswell as the amount of volume available for the internal chambers. Theacoustic chambers or passageways may have a uniform circularcross-sectional shape, or the cross-sectional shape may be quadrilateralof other geometry. For example, the cross-sectional shapes may besquare, octagonal, hexagonal, diamond shaped, or irregularly shapes. Thecross-sectional shape may be chosen to provide optimal use of theavailable internal space for a particular application. Thecross-sectional area of the chambers/passageways may be substantiallyuniform along the length of the passageway, or it may increase ordecrease. The specific orientations of the chambers 60 is typically notat all critical with respect to acoustic effects, and the orientation ofthe internal chambers 60 is largely driven by the need to fit thedesired chamber lengths within the limited volume. The fact that most ofthe chambers 60 of FIG. 3 are nearly perpendicular to the surface at thesurface was chosen to simplify the packaging requirement.

With further reference to FIG. 4, flap structure 10 may include aplurality of acoustic chambers 60, each having an opening 62. Theacoustic chambers 60 may form an upper acoustic liner 34, a loweracoustic liner 38, and/or an end acoustic liner 36 (not shown in FIG.4). Each of the acoustic chambers 60 may have a different length and/orshape to thereby absorb sound at different frequencies. Each chamber 60may be configured to behave as a quarter-wavelength resonator (sometimescalled an organ-pipe resonator). Thus, the different lengths of thechambers 60 are selected for optimal absorption of differentfrequencies. By proper selection of the combination of lengths (in ageneral sense, length is measured along the center line of the chamber,whether that is a straight path or it includes one or more bends), abroadband sound absorber can be achieved. Also, the chambers 60 withinflap 10 (FIG. 4) may be connected by optional internal passageways 72,such that sound enters through the porous surfaces and travel inmultiple directions within the flap 10. Chambers or passageways 60 mayhave opposite ends that are both open on either the upper surface or thelower surface of flap 10, or they may have one end that is open on thetop side of flap 10, and an opposite end that is open on the bottom offlap 10. For example, the chambers 60F and 60G (FIG. 4) could comprise asingle chamber extending all the way through flap 10 whereby noise istransmitted internally from the lower surface 12 of flap 10 to the uppersurface 11 of flap 10, and/or visa-versa. Designing chambers orpassageways 60 such that sound travels in multiple directions within theflap 10 via interconnecting passageways, and exits the flap 10 at adifferent portion of the porous surface represents one way to providebroadband sound absorption and/or dampening of hydrodynamic fluctuations(i.e. to reduce efficiency of their conversion to propagating sound).

The internal chambers 60 (or the entire flap interior) can also befilled with foam or other acoustic filler material, which changes themanner in which sound is absorbed as it travels through the flap 10.

The flap 10 may include a single internal chamber 60, a plurality ofsubstantially identical internal chambers 60, or a plurality of internalchambers 60 having different lengths and/or shapes. If a number ofvariable-depth chambers, separated by impervious partitions andterminated within the body of the flap, are imbedded within the flapside edge 21 (or 23), a local-reacting liner results. In theconfiguration shown in FIG. 4, the internal chambers 60 (or channels)have a circular cross-sectional shape with relatively small diameters,such that a large number of the internal chambers 60 can fit within therelatively small volume of flap 10. According to other aspects of thepresent invention, the diameter of the interior chambers can beincreased, with a porous face sheet covering the resultant openings atthe porous surface to provide the desired acoustic resistance. A largenumber of configurations may be utilized to achieve similar surfaceacoustic impedance boundary conditions by varying the geometry of theinternal chambers and the surface face sheet.

With further reference to FIG. 5, a flap 10A according to another aspectof the present invention may include upper and lower surfaces 48A and55A that are porous or have porous portions. Internal space 80 of flap10A is partially or completely filled with filler material such as foam82. Foam 82 may comprise metallic foam or other suitable material. Theupper and lower surfaces 48A and 55A, respectively may comprise thinupper and lower sheets of material 84 and 86 having a plurality ofperforations therethrough. Sheets of material 84 and 86 may be aluminum,fiber composites, or other suitable material.

The acoustic liners may also comprise extended-reaction liners. Forexample, the internal volume of the flap side edge 21 (or 23) may befilled with a bulk material such as foam, and allowing communicationbetween the interior and exterior of the flap side edge via a poroussurface such as a perforated face sheet, wire mesh, or the like.

Because of the porous nature of one or more segments of the flap surfacenear the side edge 21 (or 22), the aeroacoustic environment outside theflap 10 can communicate with the chamber or chambers 60 within the flap10. As discussed above, the interior volume of the flap 10 contains oneor more chambers 60, which may or may not be filled with sound-absorbentacoustic material such as a porous bulk material such as, but notlimited to, foam or the like. The acoustic treatment imbedded within thevolume of the flap 10 changes the boundary condition at the surface ofthe flap 10, such that the strength of the local hydrodynamicfluctuations associated with the scrubbing of the unsteady flow over theside edge surface is reduced. Furthermore, the change in the boundarycondition also inhibits the conversion of hydrodynamic fluctuations intonoise, and also inhibits near field propagation of this noise. Theboundary condition presented at the porous surface is such that itinhibits sound from being generated by the flow interaction with thissurface.

Software design tools are available to assist in the design of theinterior chambers 60 of the flap 10. Known software previously utilizedfor design of acoustic liners in engine nacelles may be utilized toassist in the design of chambers 60. Such software may also be modifiedsomewhat to thereby adapt it for use in designing chambers 60 in flap10. The use of these design tools allows the efficient design ofacoustic liners with multiple chambers, each of which can be designedwith unique geometries. This design tool also allows a convenientevaluation of configurations designed to fit within a small volume,while exploring combinations of chambers that result in broadband noisereduction.

FIGS. 6 through 11 illustrate a further embodiment, in which anorthotropic lattice structure interconnects perforated (porous) surfacesof the flap with internal lattice-structured perforations to equalizethe steady pressure field on the flap surfaces adjacent to the flap endand to reduce the amplitude of the fluctuations in the flow field nearthe flap end. Referring to FIG. 6, flap 10 comprises perforated upperside 28, perforated lower side 30, and perforated end 22 (or 20).However, the invention is not limited thereto. As examples, upper side28 and one or both ends 20 and 22 may be perforated, or lower side 30and one or both ends 20 and 22 may be perforated. At least one of ends20 and 22 is perforated, but perforations of both may be needed toproduce the desired noise reduction. All of perforations 101 integratewith internal orthogonal (approximately 90 degrees) lattice-structuredperforations 180 (shown in FIGS. 8 and 9). The perforations 101 of theflap surfaces near the end portion 40 enable the aeroacousticenvironment outside the flap 10 to communicate with the orthotropiclattice-structured perforations 180 within the flap 10. Pressure drivenflow enters into the interior orthotropic lattice-structuredperforations 180 through the porosity (perforations 101) of the outersurfaces of end portion 40. Such perforated outer surfaces can includethe various combinations discussed above. Via the interconnectivity ofinternal and surface porosity, the high pressure flow entering the flapinterior gravitates toward regions of lower pressure on the flapexterior surfaces. The high viscous damping of the internallattice-structured perforations 180 provides a strong mechanism for (1)equalizing the steady pressure field on the surfaces adjacent the endportion 40; and (2) reducing the amplitude of the fluctuations in theflow field near the end portion 40.

Looking further at FIG. 9, the perforations 182A, 182B and 182C internalto the flap 10 change the boundary conditions at the surface of theflap, significantly reducing the steady pressure differentialexperienced by the end portion 40. Such surfaces include any exteriorflap surfaces near the flap tip that have perforations, and couldinclude upper side 28, lower side 30, end 20, end 22, and anycombination thereof. As a result, the vortex formation process at theend portion 40 is either delayed or substantially reduced in such a waythat the strength of the local hydrodynamic fluctuations associated withscrubbing of the unsteady flow over the end portion 40 surfaces isdiminished. Furthermore, the change in boundary conditions is alsoexpected to inhibit the conversion of hydrodynamic fluctuations to noiseand the near field propagation of this noise. As shown in FIG. 6, theperforations 101 do not extend within trailing edge region 104A andleading edge region 104B due to the thinness of the trailing edge andstructural interface of the flap 10 to the wing 3, respectively;however, porosity could extend to regions 104A and 104B to the extentsuch regions are exposed to flow and end portion 40 structurally couldaccommodate the presence of perforations.

As shown in FIGS. 8 and 9, the internal orthogonal lattice-structuredperforations 180 comprise perforations that are orthogonally aligned inthree directions, with each approximately perpendicular to itscorresponding surface of the flap 10. One set of perforations 182Aconnects the flap upper side 28 and lower side 30. A second set ofperforations 182B extends into the flap 10 from end 20 (and/or end 22),and the maximum distance the perforations extend inboard scalesapproximately with the flap edge maximum thickness t_(max) (illustratedin FIG. 7). The third set of perforations 182C is oriented chordwise,generally following the chordwise curve of the flap 10, and does notintersect any of the exterior surfaces of end portion 40. Whileperforations in upper side 28, lower side 30 and end 22 are shown, asearlier provided, porosity could be used in a more limited manner, suchas limited to upper side 28 and end 20 or lower side 30 and end 22.Additionally, one or both of ends 20 and 22 could be porous.

As illustrated in FIG. 9, the perforations 101 are positioned such thatmember perforations from each group 182A, 182B, and 183C form a commonintersection 190 (shown enlarged in FIG. 11), thereby creating athree-dimensional lattice structure that is approximately orthotropic(approximately 90 degrees) in nature. This configuration allows everyperforation 101 to be in communication with all other perforations inthe lattice volume. Numerous variations can be made to the latticeconfiguration, and the lattice-structure and surface porosity are notlimited to the configurations illustrated. While the perforationsillustrated are octagonal in shape, the invention is not limitedthereto. Different shapes and sizes of perforations, as well asarrangements, can be utilized and can be tailored based onconsiderations such as ease of manufacturing, desired acoustic results,and minimal aerodynamic impact. As an example, the sizes of theperforations could progress from smaller to larger moving from theleading edge 13 of the flap 10 to the trailing edge 14 of the flap 10.As a further example, the spacing between adjacent perforations couldalso vary in the chordwise direction. Additionally, the perforationscould vary in dimension along their length.

Viscous diffusion via the surface perforations 101 and internalperforations 180 is significant, affecting the steady component of theflow at the end portion 40 substantially. However, the losses inherentin this configuration are less than those experienced with conventionalbulk liners. The global communication that exists within all of theperforations provides the mechanism to lessen the pressure gradientsexperienced by the end portion 40. This creates the desired alterationto both the steady and unsteady (fluctuating) components of the flowfield at the end portion 40. In addition to having diffusive effects(diffusing the incoming flow), the three-dimensional orthogonal latticestructure is also reactive (acoustic wave phase distortion) due to theinterconnection of the perforations.

The flow “reaction” produced by the orthogonal lattice design isdetermined by three parameters: perforation diameter d (shown in FIG.10) or an equivalent length scale based on the area of the hole if anon-circular shape is chosen, perforation spacing s (shown in FIG. 10),and the extent of the surface area on upper side 28, end 20, end 22, andlower side 30 having porosity. Perforation spacing s is the distancebetween the centers of two adjacent perforations 101/182A/182B/183C. Theratio of perforation spacing, s, to the perforation diameter, d, willtypically fall within the approximate range 1.4<s/d<2.0. A ratio ofs/d=1 indicates no spacing between adjacent perforations. A ratiogreater than approximately 2.0 will generally not be sufficient porosityto achieve desired flow diffusion. The dimension of the perforations101/182A/182B/182C is chosen such that the surface porosity (open arearatio per unit area) of the treated surfaces of the end portion 40 isless than approximately 40%. A surface porosity greater than this valuewill generally result in undesirable aerodynamic loss. Software designtools for computational flow simulation, such as the PowerFLOW® softwareor other commercially available software, can be used to assist in thedesign of the surface perforations and internal orthogonallattice-structure perforations to optimize noise reduction withno/minimal/acceptable impact to aerodynamic loss.

The effectiveness of the ROLD (Reactive Orthotropic Lattice Diffuser)treatment (surface porosity in communication with internal orthogonallattice-structure perforations) in substantially reducing flap noise hasbeen demonstrated via high-fidelity computational simulations, and isillustrated in FIG. 12. Porosity was used on upper side 20, lower side30, end 20 and end 22 along with internal orthogonal lattice-structureperforations in an in-service aircraft in a landing configuration. Atthe inboard tip, an effective diameter of 0.065 in and a spacing of 0.1in. At the outboard tip, a diameter of 0.052 in and a spacing of 0.080in. was used. A comparison of the fluctuating surface pressures at endportion 40 of both end 20 and end 22 between the baseline (untreated)and ROLD (treated) flaps showed an order of magnitude reduction in thefluctuation amplitudes across the entire frequency range. Moreimportantly, significant noise reduction in the far field was realized.The acoustic signatures of the baseline (untreated) and ROLD (treated)aircraft at a far field observer location are compared in FIG. 12. Notefrom the figure the effectiveness over the entire frequency range. Inparticular, in the all important frequency range of 200 Hz-4.5 kHz, theamplitude of the noise in the flyover (overhead) direction arriving toan observer on the ground is reduced by 5-6 dB. The accuracy of thecomputed results has been analyzed and the 5-6 dB gain in noisereduction is considered to have an error of ±0.5 dB.

Although the ROLD embodiment has been described in relation to a flap 10of aircraft 1, the ROLD embodiment is not limited thereto and can beapplied to any lift producing isolated airfoil tip. Examples includehelicopters (use on rotor blade tips), as well as turbine blades. It hasbroad application to any system in which there is a desire to reducenoise generated by unsteady flow at any lift producing isolated airfoiltip.

It is to be understood that variations and modifications can be made onthe aforementioned structure without departing from the concepts of thepresent invention, and further it is to be understood that such conceptsare intended to be covered by the following claims unless these claimsby their language expressly state otherwise.

The invention claimed is:
 1. A flap of the type that is movablyconnected to an aircraft wing to provide control of an aircraft inflight, the flap comprising: an elongated flap structure defininginboard and outboard ends and forward and rearward portions extendingbetween the inboard and outboard ends, the elongated flap structuredefining an upper side extending between the inboard and outboard endsand between the forward and rearward portions, and a lower side, whereinthe upper side generally faces upwardly in use, and the lower sidegenerally faces downwardly in use, and wherein the inboard end definesan inwardly-facing inboard end surface and the outboard end defines anoutwardly-facing outboard end surface, and wherein the upper and lowersides define inboard and outboard end portions adjacent the inboard andoutboard end surfaces, respectively, that tend to generate noise to forma noise source in use, and wherein: at least a portion of at least oneof the inboard and outboard end surfaces and at least one of the inboardand outboard end portions of the upper and lower sides include aperforated portion having perforations that are connected tocorresponding perforations internal to the flap, wherein the internalperforations are located within at least one of the inboard and outboardend portions, wherein the internal perforations comprise anapproximately orthogonal lattice arrangement, and wherein the connectingcorresponding perforations comprise the substantially samecross-sectional shape and dimensions.
 2. The flap of claim 1, whereinthe internal perforations comprise perforations extending into the flapfrom at least one of the inboard and outboard ends, wherein suchextensions scale approximately with the flap edge maximum thicknesst_(max).
 3. The flap of claim 1, wherein all perforations comprise thesubstantially same cross-sectional shape and dimensions.
 4. The flap ofclaim 1, wherein all perforations comprise a substantially uniformrepeating pattern.
 5. The flap of claim 1, wherein the perforations forthe at least a portion of at least one of the inboard and outboard endsurfaces and the at least one of the inboard and outboard end portionsof the upper and lower sides define a non-uniform pattern.
 6. The flapof claim 1, wherein the perforations for the at least a portion of atleast one of the inboard and outboard end surfaces and the at least oneof the inboard and outboard end portions of the upper and lower sidesvary in at least one of cross-section shape and dimensions.
 7. The flapof claim 1, wherein all perforations have a substantially octagonalcross-sectional shape.
 8. The flap of claim 1, wherein one or moreperforations has a cross-section which varies in size along its length.9. The flap of claim 1, wherein perforations for the at least a portionof at least one of the inboard and outboard end surfaces and the atleast one of the inboard and outboard end portions of the upper andlower sides comprise a porosity of less than approximately 40 percent.10. The flap of claim 1, wherein the global communication between allperforations lessens the pressure gradients experienced by the inboardand outboard ends of the flap, and alters both the steady and unsteadycomponents of the flow field at the inboard and outboard ends of theflap.
 11. The flap of claim 1, wherein the global communication betweenall perforations diffuses the incoming flow around the inboard andoutboard ends of the flap and distorts the acoustic wave phase.
 12. Theflap of claim 1, wherein the perforations for the at least a portion ofat least one of the inboard and outboard end surfaces and the at leastone of the inboard and outboard end portions of the upper and lowersides increase in size from the forward portion to the rearward portion.13. The flap of claim 12, wherein the perforations have a substantiallyuniform diameter.
 14. The flap of claim 1, wherein all perforations havea substantially circular cross-sectional shape.
 15. The flap of claim14, wherein s/d, the ratio of perforation spacing s between the centersof two adjacent perforations to the perforation diameter d, is withinthe range of approximately 1.4<s/d<approximately 2.0.
 16. The flap ofclaim 14, wherein all perforations have the substantially samedimension.
 17. A method of reducing aircraft noise, the methodcomprising: determining at least one frequency of excess noisegenerated, at least in part, at an end of a wing flap when the wing flapis in a deployed position; determining a specific geometry ofperforations in at least a portion of at least one of an inboard andoutboard end surface and at least one of an inboard and outboard endportion of the upper and lower sides, and the geometry of correspondingconnecting perforations internal to the flap and located within at leastone of the inboard and outboard end portions, wherein the internalperforations comprise an approximately orthogonal lattice arrangement,that will, in use, absorb at least a substantial portion of sound at thefrequency of the excess noise; and providing the determined specificgeometry in the flap.
 18. A method of reducing noise in a lift producingisolated airfoil tip, the method comprising: determining at least onefrequency of excess noise generated, at least in part, at an end of theairfoil; determining a specific geometry of perforations in surfaces ofthe airfoil and corresponding connecting perforations internal to theairfoil, wherein all perforations are in global communication, furtherwherein the internal perforations comprise an approximately orthogonallattice arrangement, that will, in use, absorb at least a substantialportion of sound at the frequency of the excess noise; and providing thedetermined specific geometry in the airfoil.